The present invention relates to the production of a semiconductor device or the like, and more particularly to a controlling method for forming a thin film suitable for stabilization of formation or treatment of a thin film of a semiconductor, a system for controlling the method, an exposure method and a system for the exposure method.
With the progress of high integration of semiconductor devices, pattern size has become finer, the device structure has become three-dimensional, and the manufacturing processes of semiconductor devices have become more complicated. It is therefore necessary to pay more attention than before to the stabilization of production process conditions in the manufacturing steps of semiconductor devices.
For instance, in projecting a pattern drawn on a reticle onto a wafer by a projection aligner, exposing light of a single wavelength in a comparatively narrow wavelength bandwidth is used. Therefore, as shown in FIG. 2a, the exposure light 71 undergoes multiple reflection in a photoresist film 72 or in a light-transmitting undercoat forming film 74. As a result, mutual interference of light occurs and the intensity of light varies in the depth direction inside the photoresist film 72. Accordingly, the exposure energy is varied in the depth direction and, upon development, the cross section of the photoresist film appears rugged, as shown in FIG. 2b. When the process conditions in various manufacturing apparatuses are varied, the thickness of photoresist film t or the formed condition of the light-transmitting undercoat 74 is varied. Consequently, upon exposing a photoresist under the same exposure energy, the width W of the photoresist in contact with the uppermost layer of the undercoat 73 is varied under the effect of stationary waves, resulting in varied pattern size. In order to stabilize the pattern size W, an optimum exposure energy according to the variation in the thickness of photoresist film t and the formed condition of the undercoat 74 should be set.
When the optical property or thickness of the photoresist film is varied due to variations in the photoresist coating or baking conditions in a photoresist coating machine, the pattern size varies even if the formed condition of the undercoat on the wafer or exposure and developing conditions are the same. It is therefore necessary, even in the photoresist coating machine, to keep monitoring the variation in coating and baking conditions, the major causes of variations in the thickness and optical properties of the photoresist film.
In thin film forming and treating steps such as the film forming step and etching step before or after the exposure step, as shown in FIG. 4, due to the increase in the diameter of the wafer formed and treated and the decrease in the thickness of film, the thickness and optical properties of the thin film formed and treated are varied with slight variations in the production process conditions. In a thin film forming and treating apparatus, therefore, it is necessary to constantly monitor the thickness and optical properties of the thin film being formed or treated, and to control the process conditions so as to keep constant the thickness and optical properties of the thin film.
A preliminary operation method of maintaining a constant pattern size in, for example, an exposure step in the presence of variations been performed in which exposure and development of one or several sheets of wafer are conducted on a trial basis. The pattern size is measured by a measuring instrument to judge the acceptability of exposure energy. The judgment is fed back to the opening and closing times for shutters in an optical system for illumination, or the like.
In the manufacture of small volumes of many types of products such as ASIC (Application Specific Integrated Circuit), however, the preliminary operation is required every time the type of product is changed. The requirement has increased the number of working steps and has been the major cause of lowering the operating efficiency of an exposure apparatus. With the trend towards higher integration, the method of correcting variations in the process conditions by such preliminary operations is unable to give sufficient accuracy.
In order to eliminate the preliminary operations, a method has been devised, as disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 63-31116 (1988). In the method, assuming that the relationship between the thickness of a photoresist film and pattern size and the relationship between exposure energy and pattern size are known, the thickness of a photoresist film on a wafer to be exposed is measured by a photoresist film thickness measuring apparatus incorporated in a reduction projection aligner. The measurement result is fed back to the exposure energy so as to reduce variations in the pattern size and to stabilize the pattern size.
In a thin film-forming and treating step, also, a method has been used in which the thickness of a photoresist film is measured for the preceding wafer by an apparatus for forming and treating a thin film. The process conditions for fabrication of the intended product is set based on the film thickness thus measured.
Of the prior arts mentioned above, the stabilization of pattern size has been carried out by measuring variations of the thickness of a photoresist film formed by coating, determining an optimum exposure energy based on the measurement results, and controlling the pattern size. With the recent increasingly higher integration of semiconductor devices, however, it has become impossible to ignore the effects of variations in the manufacturing process conditions, such as variations of the formed state of an undercoat due to variations in forming and treating conditions in the thin film forming and treating step, variations of the optical properties of a photoresist film due to variations in coating and baking conditions in the photoresist coating step, etc.
Furthermore, in the thin film forming and treating step according to the prior art, the operation with the current film thickness measuring apparatus is affected directly by variations in the state of an undercoat formed in the precedent step resulting in errors in the measured values of the film thickness. It is therefore difficult to set accurately the optimum process conditions based on the measurements of the film thickness.